What is temperature?

Is today a jacket day or a T-shirt day? Is the bath too hot? Do you have a fever? To answer questions like these, we need to know the temperature: how hot or cold something is.

What makes something hot or cold?

Everything is made of tiny particles: atoms, and little groups of atoms stuck together. They are far too small to see, even with an ordinary microscope.

These particles never stop moving. In a solid, like an ice cube or a spoon, they jiggle in place. In a liquid, like water, they jiggle and slide past each other. In a gas, like air, they zoom around and bump into each other.

When something gets warmer, its particles jiggle faster. When it cools down, they slow down. So temperature tells how fast the particles are jiggling: the faster they jiggle, the higher the temperature.

How do we measure temperature?

We measure temperature with a thermometer. Many thermometers are thin glass tubes with a coloured liquid inside.

When the liquid warms up, it takes up a little more space, so it climbs up the tube. When it cools, it shrinks back down. The numbers on the side show the temperature.

Many digital thermometers have a tiny electronic part at the tip instead. Electricity flows through it more easily or less easily, depending on how warm it is. A small chip turns this into a number on the screen.

Degrees Celsius

In most countries, we measure temperature in degrees Celsius, written °C. The scale uses water as its guide:

• At 0 °C, water freezes into ice, and ice melts.
• At 100 °C, water boils, at least at sea level.

On a high mountain, water boils at a lower temperature. Find out why in why water boils at different temperatures.

Temperatures colder than freezing get a minus sign. So −18 °C means 18 degrees below freezing.

The scale is named after Anders Celsius, a Swedish astronomer. He described it in 1742.

From the freezer to the Sun

Here are some temperatures to compare:

What	Temperature
The coldest air ever measured on Earth, in Antarctica	about −89 °C
Inside a freezer	−18 °C
Water freezes, ice melts	0 °C
Inside a fridge	about 4 °C
A comfortable room	about 20 °C
Your body	about 37 °C
Water boils (at sea level)	100 °C
A hot oven	about 200 °C
The surface of the Sun	about 5,500 °C

Your body keeps itself at about 37 °C, in summer and in winter. When you are ill, it can climb a little higher. That is a fever.

An experiment: metal and wood

Here is a safe experiment with a surprise. Find a metal spoon and a wooden spoon that have been lying in the same room for at least an hour. Now they both have the same temperature as the room.

Touch them, one after the other. The metal spoon feels much colder! But a thermometer would show the same temperature for both. How can that be?

Your hand is warmer than the room, so heat flows out of your hand into anything you touch. Metal passes heat along very quickly. It pulls heat out of your skin fast, and your skin cools down. Wood passes heat along slowly, so your skin stays warm.

Your skin does not feel the temperature of the thing you touch. It feels how fast it is losing heat. Fast means “cold!”, and slow means “not so cold”.

Go deeper: Degrees Fahrenheit for grown-ups and the extra curious

In the United States, people measure temperature in degrees Fahrenheit (°F). On this scale, water freezes at 32 °F and boils at 212 °F, and a healthy body is about 98.6 °F. To change degrees Celsius into degrees Fahrenheit:

$$F = C \cdot \frac{9}{5} + 32$$

Here $C$ is the temperature in degrees Celsius and $F$ is the same temperature in degrees Fahrenheit. A room at 20 °C is $20 \cdot \frac{9}{5} + 32 = 68$ °F.

Go deeper: Kelvin and absolute zero for grown-ups and the extra curious

Can things get colder and colder forever? No. The colder something gets, the slower its particles jiggle, and there is a limit: absolute zero, at −273.15 °C. There, the particles have the smallest jiggle possible. They cannot give away any more energy, so nothing can ever be colder. (Quantum physics says that a tiny jiggle remains, even at absolute zero.)

Scientists often use the kelvin scale (K), which starts at absolute zero. One kelvin is the same step as one degree Celsius:

$$T_{\text{K}} = T_{\text{C}} + 273.15$$

Here $T_{\text{C}}$ is the temperature in degrees Celsius and $T_{\text{K}}$ is the same temperature in kelvins. Water freezes at 273.15 K, a room is about 293 K, and the surface of the Sun is about 5,800 K. Kelvins are written without the degree sign.

Nobody has ever reached absolute zero. But scientists have cooled atoms to less than a billionth of a degree above it.

Go deeper: What temperature measures, precisely for grown-ups and the extra curious

Temperature is linked to the average movement energy of the particles, not to their total energy. That is why a bathtub of warm water holds far more heat energy than a cup of hot tea: the tea’s particles move faster, but the bath has many more of them.

Our skin is a poor thermometer. It senses its own temperature, which drops quickly when a material draws heat away fast. Metals and stone do this strongly; wood, cork and wool hardly at all. Engineers call this property thermal effusivity.

Check yourself

A metal spoon and a wooden spoon have been lying in the same room all day. Which one is colder?

The metal spoon The wooden spoon Neither: they have the same temperature

Show the answer

Neither: they have the same temperature — Both have the temperature of the room. The metal only feels colder because it pulls heat out of your hand faster.